The present invention relates to charge storage cathode ray electron devices, and more particularly to an improvement to the charge storage target of the device.
Prior art charge storage targets comprise a conductive layer and a perforated insulative layer in contact with the conductive layer. During writing operation, an intensity-modulated electron beam is permitted to impinge upon the surface of the insulative layer as well as the underlying conductive layer. As a result negative charges are accumulated on the insulative layer to form an electron image. During reading operation, an unmodulated electron beam scans across the target face. Because of the action of the negative surface potential on the insulative layer, the path of impinging electron beam is deflected as a function of the amount of charges on the insulative layer so that the electrons that impinge on the underlying conductive layer is intensity modulated by the negative surface potential, resulting in generation of a video signal from the conductive layer. Because of the direct impingement of electrons on the conductive layer, the operating characteristic of the prior art charge storage target, as illustrated in FIG. 1, has a small range of negative surface potentials at the insulative layer for a given range of video output currents. This means that for a given amount of output current variations, the available range of surface potential variations is limited and as a result gradual shading of image, or gradation is difficult to achieve. Furthermore, a surface noise on the insulative layer would produce a signal-to-noise ratio of comparatively small value, typically as high as 20 dB for a surface noise of 0.1 volts.